1. Field of the Invention
This invention relates to fluid transfer control apparatus and, particularly, to the use of optically-based overfill probes for detecting when fluid being transferred into a container has reached a predetermined level.
2. Description of the Related Art
In the art of fluid transfer control, particularly as it applies to the petroleum industry, one of the more common control devices is an overfill sensor for determining when the fluid being transferred into a container, such as a petroleum tanker compartment, has reached a predetermined level. An output signal from such a probe indicates when the fluid has reached the predetermined level, and may be used as an indication by a fluid transfer controller to discontinue fluid flow into the container. In this way, overfilling of the container, which is particularly hazardous when dealing with flammable liquids such as gasoline, can be avoided.
One type of overfill probe which is particularly common in the petrochemical industry makes use of an optical signal which is coupled into a medium having a relatively high index of refraction, such as a glass or non-opaque plastic. This medium is specially-shaped and commonly referred to as a xe2x80x9cprism.xe2x80x9d The prism is shaped to cause internal reflection of the optical signal when surrounded by air. The shape of the prism and the direction at which the optical signal is coupled into the prism is such that the reflection of the optical signal within the prism redirects the signal toward a photodetector. This photodetector generates an output signal which indicates that the optical signal is being detected.
A schematic illustration of this prior art probe design is shown in FIG. 1. In the plane of the optical signal path, the prism 10 has a triangular cross section. The optical signal is generated by light source 12. When the prism 10 is surrounded by air, the optical signal (indicated by the arrows in FIG. 1) is reflected at two interfaces between the prism material and the surrounding air, and redirected toward photodetector 14. The photodetector 14 generates an electrical output signal which indicates that the optical signal is being detected.
As shown in FIG. 1, the prior art prism 10 uses a forty-five degree incidence angle (relative to normal) for each of the reflections of the optical signal within the prism 10. This prism 10 has the triangular cross section shown, and light source 12 and photodetector 14 are oriented in the same direction along the same surface of the prism 10. When in use, the prism is part of a probe which is located within a fluid container, usually near the top of the container. When the fluid in the container rises high enough to contact a prism surface at a location where the optical signal is incident, the forty-five degree angle is no longer sufficient to provide internal reflection of the optical signal at that interface. This is because the prism/air interface becomes a prism/fluid interface, and the fluid has an index of refraction much closer to the prism material than does air. According to Snell""s law of refraction, (well-known in the art of optical design) the forty-five degree angle of incidence of the optical signal now results in the transmission of the optical signal through the interface due to the similarity of the relative indices of refraction. As a result, the signal is no longer detected by photodetector 14, and the corresponding change in the photodetector output signal is used to discontinue loading of the container.
One of the problems encountered with a prior art probe such as that shown in FIG. 1 is related to the operational temperature range of the probe. When the probe is used in cold ambient temperatures (common for a petroleum tanker truck which has the probe within one of its tanker compartments and which delivers fuel in regions having relatively cold climates), is that condensation, or even frost, may form on the external surfaces of the prism. If sufficient condensation forms on the prism when the fluid level in the container is below that at which it should be detected by the probe, the condensation may nonetheless cause transmission of a significant portion of the optical signal through the surface of the prism. This portion of the signal then goes undetected by the photodetector. If the signal loss is high enough, the signal detected by the photodetector (and indicated by the photodetector output signal) may be below the detection threshold used to indicate when the fluid in the container has reached the probe level. As a result, a false overfill signal may result which prevents fluid from being loaded into the container, despite the fact that the container may be empty.
In the past, one of the solutions to the condensation problem has been to increase the sensitivity of the photodetector so that it is activated by smaller amounts of reflected light. However, this also makes the probe more sensitive to inadvertent reflections from surfaces within the container. When the prism is in contact with the fluid, the light from the light source can pass through the fluid, be reflected off a reflective surface within the container, and find its way back to the photodetector. If the reflected signal is strong enough, this can result in a dangerous overfill situation, as the contact of the prism by the fluid goes undetected, and the container continues to be filled to the point of overflowing.
The improved overfill probe of the present invention makes use of an optical signal in the infrared (IR) range, generated from an IR source, such as an diode having an output in the IR range. The optical signal is coupled into a first medium of fluoropolymer, in the preferred embodiment TEFLON Perfluoro Alkoxy (TEFLON PFA), although other fluoropolymers may also be used. TEFLON PFA is manufactured by, and TEFLON(copyright) is a registered trademark of, E.I. du Pont de Nemours and Co., Inc. The prism has a particular shape which results in the internal reflection of the IR signal when the reflecting surfaces are contacted by a second medium (e.g. air) having an index of refraction significantly lower than that of the prism material. The reflection of the optical signal is toward a photodetector of the probe, which detects the optical signal and generates an output signal in response thereto.
The probe is located in a fluid container, such as a compartment of a petroleum tanker truck, with the prism positioned such that it is contacted by fluid in the container when the fluid is at a predetermined fluid level. The optical signal from the IR source is coupled into the prism and, while the fluid level is below the predetermined level (i.e. while the probe is surrounded by the second medium), the optical signal is reflected by at least one interface between the prism and the second medium. The optical signal is ultimately directed toward the photodetector through internal reflection within the prism. As the container is filled with liquid, the fluid level rises toward the prism. When the fluid reaches the prism, the new optical interface formed by the prism and the fluid allows transmission of the optical signal through the interface. Without the reflection of the optical signal, the signal is no longer detected by the photodetector. As a result, the output signal of the photodetector changes, indicating that the optical signal is no longer detected, and the change can be used by a fluid transfer controller to discontinue fluid transfer into the container, thereby preventing overfilling.
In addition to the unique material of the probe prism of the present invention, the prism is also a unique shape. In particular, the prism has a cross-sectional shape which is preferably substantially a quadrilateral. This cross-sectional shape results in the light source and photodetector not being oriented in the same direction, but also provides a higher angle of incidence (relative to normal) of the optical signal on the internally reflective surfaces of the prism. There is therefore less chance of optical leakage through the reflective surfaces of the prism due to localized surface irregularities (i.e. due to the surface not being perfectly smooth on a microscopic level, as may result from moisture or frost) and better overall signal performance.
The present invention also includes a two-piece probe housing which screws together to enclose the probe components. The tightening together of the housing portions compresses an outwardly-extending portion of the prism material, firmly holding it in place. The compression of this material also causes it to flow outwardly, sealing it against an inner surface of the housing. The housing also includes a lower portion which is roughly cylindrical with cutouts along its surface. The cutouts allow air to escape as fluid rises within the cylindrical portion. The surfaces of the lower portion surrounding the cutouts prevent light from the light source from being inadvertently reflected off a reflective surface within the container back to the photodetector when fluid in the container is in contact with the prism.